High purity dicumyl peroxide is used as a catalyst in a variety of chemical reactions. One of the most important uses of dicumyl peroxide is as a cross-linking agent for polyethylene compounds such as those used for wire in cable coating. In addition, dicumyl peroxide is also used for cross-linking ethylene-vinyl acetate copolymer compounds. In order to be suitable for these uses, the dicumyl peroxide must be of a very high purity, at least 90%, and free from contaminants and by-products of production.
Crude dicumyl peroxide has been prepared by various methods. These methods generally comprise reacting cumene hydroperoxide with either cumyl alcohol or alpha-methyl styrene. Since most, if not all, of the commercial methods employ the use of acidic type catalysts, the crude dicumyl peroxide contains impurities such as acetone, phenol, acetophenone and unreacted cumyl alcohol or alpha-methyl styrene. Since it is not practical to purify the cumene hydroperoxide starting material above about 80 to 85%, the cumene hydroperoxide itself contains not only unreacted cumene but also some relatively high boiling impurities. These include cumyl alcohol, acetophenone and other materials which are carried over into the dicumyl peroxide.
Prior art, which is exemplified by British Pat. No. 1,243,313, teaches that acetone and phenol may be removed from crude dicumyl peroxide by first extracting the material with an aqueous sodium hydroxide solution and followed by washing with water to remove the alkaline solution. The final washed product may then be purified by several methods which include crystallization and recrystallization, heating under vacuum to remove the volatile impurities and a partial steam distillation under vacuum. The latter method, under reduced pressure, is a much better method of removing the volatile impurities.
Both of the above distillation methods of removing the volatile impurities; that is, the vacuum distillation in the absence and in the presence of added water, have been evaluated and both do, in fact, remove some of the volatile impurities. However, both methods have significant disadvantages. The principal disadvantage of the vacuum distillation in the absence of water is that a vacuum in the order of one millimeter of mercury is required at a temperature of about 90.degree. to 100.degree. C. for a prolonged period of time in order to produce a product of acceptable purity, i.e. at least 90%. Although this method may be accomplished batch-wise in the laboratory, there is extreme inherent danger in heating large amounts of dicumyl peroxide at such an elevated temperature. Since dicumyl peroxide is not thermally stable, some decomposition occurs during this long heating process.
British Pat. No. 1,243,313 states that "the other method" (other than crystallization) "for purifying crude dicumyl peroxide consists of distilling impurities by heating the crude peroxide at 90.degree. to 110.degree. C. under high vacuum for several hours. The hazards of heating peroxides at these temperatures and the necessity of a high vacuum make this process commercially unattractive. This distillation can give a product assaying about 90% but it has a dark amber color and must be decolorized and recrystallized to be acceptable commercially; also these products may go off color in ordinary storage". From this patent, and from our own laboratory work, it became obvious that an improved method for purifying dicumyl peroxide was needed in order to rapidly produce a high purity product for commercial use.
We have now discovered an improved method that will rapidly purify crude dicumyl peroxide. The dicumyl peroxide obtained from our process not only is high in purity, 90 to 95.+-.%, but is also light in color. Our process allows for both batch-wise and continuous process of the product with equal purity.